The world is facing an unprecedented crisis in the form of COVID–19 (coronavirus, SARS-CoV–2 disease) pandemic, a highly infectious disease. As of 30th May, 2020, more than 6 million cases and 365k deaths1 indicate the severity of the current situation. The modes of transmission for SARS-CoV–2 in populations include droplets, aerosols and physical contact, very similar to some recent pandemics (Severe acute respiratory syndrome (SARS)–1, Middle east respiratory syndrome (MERS), Influenza) 2–4. Wearing of face mask (or filtering facepiece respirator: FFR) is one of the key public recommendation made by the scientists5, designated authorities and governments. This is posed as a last defense to control the transmission from the infected to healthy individuals, implying better public health. Although the research studies on face masks have been consistent, resurgence is seen during the pandemic period6, periodically bringing the focus on aspects related to the effectiveness of face masks in controlling the airborne transmissions.
A face mask is a combination of aerosol filters and/or fabrics designed to reduce the concentration of inhaled particulates to which the wearer is exposed. Other factors crucial to its design are pressure drop, moisture absorption, reliability, reusability and comfort fit7,8. There are different nomenclatures to specify face mask designs as per their characteristics and targeted users. Non-powered air purifying particulate respirators are segregated as N-series (for workplace free of oil aerosols) and R- & P- series (exposure includes oil-based liquid particulates) respirators9. N-series respirators are classified as per their capture efficiency for the most penetrating particle size (MPPS) of 0.3 µm. Thus N–95 class refers to 95 % capture efficiency for the MPPS at the testing conditions specified as per the documented protocols given by certifying agencies. FFRs (or filtering face piece: FFPs) are also classified as FFR1 (80 %), FFR2 (94%) and FFR3 (99%) designs.
Respirators remove particles from the inhalable stream via aerosol filtration processes i.e. interception, diffusion, gravitational settling, impaction and electrostatic deposition. The efficiency of removal depends on aerosol characteristics viz. concentration, particle size, charge and flow rate of the air stream. In terms of particle capture considerations, decrease in fiber diameter and increase in respirator thickness is sought from face mask designs10. A balance between the quality factor11 and other issues such as compromising breathability, availability of the raw material etc. finally determines the choice amongst the options of traditional facemasks, surgical masks and nanofiber FFRs12. Several scientific bodies recommend guidelines to be followed for the use and certification of the face masks. For example, in United States of America, Occupational Safety and Health Administration (OSHA) recommends a compliance program where the use of National Institute for Occupational Safety and Health (NIOSH)-approved respirator is mandated. Similarly, standard testing procedures in terms of documents and protocols are in place worldwide (American Society for Testing and Materials standards, Bureau of Indian standards, European Union protocols, Korean Food and Drug Administration protocols etc.). The various filtration test methods differ in terms of aerosol type, flow rate/face velocity and sample type and extensive studies have been carried out on optimizing effectiveness of different types of face masks under these guidelines. The effectiveness of N–95 FFRs and surgical mask in health care setting has been extensively reviewed13,14. As per the convenience and due to the concept of MPPS (generally in the range of 0.2 to 0.5 m) for aerosol filtration, optical based detection (> 0.2 µm particle size) has been preferred in these test methods. As infected individuals release considerable numbers of submicron particles in the exhaled breath15,16, detailed measurements in this size range have also been reported17,18 . These mostly include electrical mobility measurements for interpretation and methodology development19–21.
Contamination during manufacturing, transportation and storage of face masks calls for availability of appropriate decontamination strategies22. Decontaminated face masks (including for re-use applications) may get degraded in terms of performance criteria. Prevention of leakages in face masks, including edge seal leakages is another crucial aspect which decides the performance of the full mask23,24. Moreover, it is important to ascertain the fit of the mask normally as well as following any decontamination treatment25,26. In the past epidemics as well as in the present COVID–19 situation, there has been a dearth of supply of N–95 masks for health care workers27 and for other essential services workers. Hence, urgent efforts are undertaken to develop new designs or to explore alternative arrangements. In past, different materials as the component of face mask have been explored w.r.t. their performance28–33. New designs need testing in a set-up which either is accessible at ease or can be made in a laboratory or industrial setting. A simple methodology to determine whether the risk of the use of designed face mask is acceptable is the utmost desirable deliverable of such a set-up.
The objective of this work is to evolve a quick methodology, which can be adopted very easily by a standard aerosol laboratory, for testing the quality of a face mask in terms of its particle (intrinsic and full mask) capture efficiency, pressure drop and the fit factor. The test conditions may not conform to a particular standard, but are more tuned towards getting first level impressions. Nevertheless, the setup can be tuned for stricter test conditions depending on the requirement. The study discusses experiments with 3 different types of face masks and enumerates the results with a view to evolve an appropriate methodology for the present context.